All of this sounds familiar to me, but I'm not the David who spoke of the stairs. Even before cancer I'd suffered occasionally from BPPV (Benign Paroxysmal Positional Vertigo), for which my neurologist prescribed, yes, the Epley Maneuver.
Here's a video to explain it:
http://www.youtube.com/watch?v=pa6t-Bpg494I found it worked beautifully for me. So yes - definitely finish off the radiation, that's key. Then try these exercises and see if they help. If not you might consider speaking with a neurologist if your other docs aren't addressing the question adequately.
Good luck - keep us posted.